Vibrating flow meters such as, for example, densitometers and Coriolis flow meters are used for measuring a characteristic of flowing substances, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating flow meters include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. The one or more conduits are vibrated by at least one driver at a resonance frequency in one of these modes for purposes of determining a characteristic of the flowing substance. One or more electronics transmit a sinusoidal driver signal to the at least one driver, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The driver signal causes the driver to vibrate the one or more conduits at the driver frequency in the driver mode. For example, the driver signal may be a periodic electrical current transmitted to the coil.
At least one pick-off detects the motion of the conduit(s) and generates a sinusoidal pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. The pick-off signal is transmitted to the one or more electronics; and according to well known principals the pick-off signal may be used by the one or more electronics to determine a characteristic of the flowing substance or adjust the driver signal, if necessary.
Typically, vibrating flow meters are provided with two vibrating conduits that vibrate in opposition to each other in order to create an inherently balanced system. As a result, the vibrations from each conduit cancel each other out in a manner that prevents vibration or torque forces from being transmitted to any connecting structures. Likewise, when two vibrating conduits are used, vibrations of the mounting structure are canceled in the flow meter because the pick-offs generally measure only relative motion between the flow tubes, and externally induced vibrations tend to vibrate both tubes equally. There are, however, certain applications where dual conduits are undesirable, for example, due to problems with pressure drops or clogging. In such situations a single conduit system may be desirable.
However desirous a single conduit system may be, single conduit systems present inherent imbalance problems. Attempts at solving this problem have involved using a balancing structure, for example, a dummy tube or a balance bar, and using the motion of the balancing structure to balance out the system. Since, however, the overall mass of the tube, including the fluid within the tube, changes as the density of the fluid within the tube changes, these techniques by themselves have received limited success at eliminating imbalance problems.
FIG. 1 depicts a single conduit type vibrating flow meter according to the prior art. As shown, the flow meter includes a case 103 enclosing a balance bar 102. The balance bar 102 is cylindrical and encloses conduit 101. Case 103 has end elements 104 coupled by neck elements 105 to input and output flanges 106. Element 107 is the input to the flow meter; element 108 is the output. Conduit 101 has an input end 109 connected to an opening in case end 104 at element 112 which is the brace bar portion of case end 104. Brace bar portion 112 is coupled to neck element 105. On the right side, the output end 110 of conduit 101 is coupled to the case end 104 at location 112 where case end 104 joins neck element 105.
In operation, conduit 101 and balance bar 102 are vibrated in phase opposition by a driver (not shown). With substance flowing, the vibration of conduit 101 in this example induces a Coriolis response in conduit 101 that is detected by pick-off sensors (not shown). The phase displacement between the pick-off sensors represents information pertaining to the flowing substance. The signal output of the velocity sensors is applied to electronics circuitry that processes the signals to derive the desired information pertaining to the flowing substance, such as for example a mass flow rate, a density, a viscosity, etc.
It is necessary that a vibrating flow meter provide accurate information over a wide range of operating conditions including substances of different density, temperature, and viscosity. In order to achieve this, it is desirable that the flow meter operate stably over a range of conditions. In order to achieve this stability, it is desirable for the flow meter vibrations to be isolated to the conduit and balance system, because vibrations external to the vibratory system, whether induced by the vibrations of the flow meter or from another source, such as a pump, imposes additional accelerations on the flowing substance besides the Coriolis acceleration used to determine the fluid characteristics of the flowing substance. External vibration also repositions the nodes (area experiencing no motion) defining the active length of conduit. This effect is difficult to compensate for and is subject to unknowable parameters such as the rigidity of the structure to which the meter is connected. Accordingly, undesired vibrations impede the ability of the flow meter to provide accurate output information regarding the flowing substance.
For the flow meter of FIG. 1, the vibrating system includes balance bar 102 and conduit 101, which are vibrated in phase opposition. These two elements comprise a dynamically balanced system in which the ends 111 of balance bar and ends 109 and 110 of the conduit 101 are coupled by brace bar portion 112 of case end 104. This is undesirable since the processing of substances of different densities may cause the vibration of the case and flanges. Because the vibration amplitude of the case 103 and flanges 106 is dependent upon the stiffness of the structure to which the meter is mounted, error of unknown magnitude can be induced in the flow measurement.
The better attempts at solving imbalance problems that arise due to changes in the density of the fluid involve adjusting the ratio of the vibration amplitude of the conduit relative to the vibration amplitude of the counterbalance structure. In other words, momentum is what is being balanced, momentum is the product of mass and velocity, and velocity is proportional to vibration amplitude. If, for example, the mass of a conduit (including the fluid located inside) and the mass of the counterbalance structure were initially equal and then the mass of the conduit were doubled (for example, as a result of a density increase in the fluid within the conduit), then reducing the amplitude of the conduit by half would restore balance to the conduit/counterbalance system. In practice, the combined amplitude of both the counterbalance structure and the conduit can be controlled by meter electronics. Accordingly, the conduit amplitude may be reduced to a lesser extent and the balance structure amplitude may be increased to some extent until in the above example, the ratio of the counterbalance amplitude relative to the conduit amplitude is 2:1.
Adjusting the amplitude in traditional methods has a significant drawback in that it results in the repositioning of motionless nodes that reside along the axis of the vibrating structure. Node relocation is a problem in flow meters because the nodes are typically located on the conduit where the balance structure joins the conduit. Accordingly, the area between the nodes usually defines the active length of the conduit. The active length affects the measurement sensitivity. Further, if the nodes reposition, then the end portions of the tube may vibrate, this further causes the flanges to vibrate. These undesirable vibrations can further affect the measurement sensitivity.
The traditional method of getting the amplitude ratio to change is to isolate the vibrating structure (conduit, balance bar, and connecting structure) with a very soft mount. The idea is that a vibrating structure isolated in space is always balanced. For example if a spring joins two equal masses in space, such that when set vibrating out of phase with each other, the masses vibrate with equal amplitude, then the spring has a motionless node half way between the masses. If one mass were to be increased and the masses were again set vibrating, the vibration amplitude of the increased mass automatically decreases, and the vibration amplitude of the other mass automatically increases to keep the momentum balanced. However, as a consequence, the new position of the node is closer to the larger mass. The vibrating structure of a vibrating flow meter is similar, and node relocation is a problem for similar reasons.
The present invention is directed to a balance system for a vibrating flow meter.